With the increasing concern regarding environmental issues in recent years, electricity storage systems that store electric power generated through the use of natural energy such as solar power generation (PV: Photovoltaic) and wind power generation have been receiving increased attention. The use of lithium ion secondary batteries that do not contain substances such as lead that are hazardous to humans or to the environment is being studied for use in these electricity storage systems.
When an electricity storage system is to be produced using lithium ion secondary batteries (hereinbelow generally referred to as “secondary batteries”), the electricity storage system is typically configured by connecting a plurality of secondary batteries in series to obtain the required output voltage to form a secondary battery pack and then connecting a plurality of secondary battery packs in parallel to obtain the required storage capacity.
However, in a configuration in which a plurality of secondary battery packs (or secondary batteries) are connected in parallel, the concern arises that imbalances of the voltage across the terminals of each of the secondary battery packs during operation will give rise to cross-currents by which current flows between secondary battery packs, rendering the electricity storage system incapable of normal operation. Differences in the state of deterioration of secondary batteries are caused by, for example, the time of manufacture or the temperature of the environment, and if differences occur in the state of deterioration even between secondary batteries of the same type (standard), an imbalance will occur in the voltage across the terminals of each secondary battery after charging and discharging, and this imbalance will bring about the occurrence of cross-currents between secondary batteries. This problem becomes more pronounced as the number of secondary battery packs (or secondary batteries) that are connected in parallel increases.
The adverse influence of cross-currents between secondary battery packs or secondary batteries upon an electricity storage system is generally known, and the mixing of old and new secondary batteries is therefore prohibited in most apparatuses that use a plurality of secondary batteries connected in parallel.
Nevertheless, in actuality, even when only new secondary batteries are used, differences in the progress of deterioration will occur between secondary batteries during operation. There are also many situations in which the combined use of secondary batteries having different states of deterioration is to be desired, such as when an electricity storage system is initially configured using only a few secondary batteries but subsequently secondary batteries are to be added, or when used secondary batteries are employed so that an electricity storage system can be configured inexpensively.
Based on this background, technology is now being sought that enables secondary batteries having different states of deterioration to be safely and freely used in an electricity storage system and thus reduces the risk to users.
As such a technology, Patent Document 1 discloses a technique in which a plurality of secondary battery packs are provided that can be connected in parallel by way of switches and that thus eliminates voltage imbalance across terminals of each secondary battery pack at the time of charging and discharging by controlling the switches that are provided for each secondary battery pack. When each secondary battery pack is to be discharged in the technique disclosed in Patent Document 1, the voltage across the terminals of each secondary battery pack is measured, discharge of voltage across the terminals is started from the secondary battery pack having the highest voltage across terminals whereby the voltage across the terminals decreases due to discharge, and when the voltage across the terminals is substantially equal to that of other secondary battery packs that have not been discharging up to this point, the discharge of the other secondary battery packs is started. When each secondary battery pack is to be charged in the technique disclosed in Patent Document 1, charging is started from the secondary battery pack having the lowest voltage across the terminals whereby the voltage across the terminals increases due to the charging, and when the voltage across the terminals is substantially equal to the voltage across the terminals of other secondary battery packs that have not been charging to this point, the charging of the other secondary battery packs is started. By means of this control, voltage difference across the terminals of each secondary battery pack at the time of starting charging and discharging can be reduced, whereby the occurrence of cross-currents can be prevented.
In the electricity storage system that is described in the above-described Patent Document 1, unless the voltage difference across terminals between a secondary battery pack that is discharging or charging and another secondary battery pack that has been subsequently added and that is to start charging or discharging is within a predetermined value, the discharging or charging cannot start in the other secondary battery pack. As a result, cases will arise in which the electricity storage system cannot be operated with flexibility because it is not possible to individually control each secondary battery pack that is discharged or charged according to, for example, the electric power that is required by the load, the power generation capability of, for example, PV systems, or the residual capacity of each secondary battery pack.
Normally, each secondary battery pack that is provided in an electricity storage system is equipped with a switch for discharging (hereinbelow referred to as the “discharging switch”) and a switch for charging (hereinbelow referred to as the “charging switch”), whereby discharging and charging can be performed individually in secondary battery pack units. In an electricity storage system that does not adopt the technique described in Patent Document 1, however, the discharging switch and charging switch of each secondary battery pack must be controlled such that the above-described cross-currents do not occur.
For example, in an electricity storage system that is provided with two secondary battery packs, a case can be considered in which the secondary battery pack that is the object of charging switches from the secondary battery pack having the higher voltage across terminals to the other secondary battery pack having the lower voltage across terminals to charge the other secondary battery pack. In order to prevent cross-currents between the secondary battery packs in this case, the discharging switch and charging switch of one secondary battery pack must each be turned OFF to briefly halt the charging operation of the electricity storage system before starting charging of the other secondary battery pack.
Here, when continuous charging using electric power that is generated by, for example, PV systems or wind power is desired with one secondary battery pack linked to the other secondary battery pack, the electric power that is generated by PV systems or wind power cannot be used for charging at the time that the secondary battery pack that is to be charged is switched, and the power generation capability therefore cannot be used to its full potential.